As a power source device which drives a large AC load such as a motor or drives a plurality of AC loads in parallel by using a DC power source as a power source, a power conversion device where a plurality of power conversion units are provided in parallel with each other is used. FIG. 3 is a schematic configurational diagram illustrating such a type of the power conversion device, and the reference numeral 1 denotes a DC power supply unit. The DC power supply unit 1 is configured with, for example, a diode circuit to rectify and smooth a commercial AC power source, a battery, or the like.
In addition, the reference numeral 2 (2a to 2n) denotes a plurality of power conversion units which are connected to the DC power supply unit 1 through a DC link portion 3 to be provided in parallel with each other. Each of the power conversion units 2 (2a to 2n) is configured with an inverter unit which converts the DC power supplied from the DC power supply unit 1 to AC power and outputs the AC power. For example, output terminals 4 (4a to 4n) of the power conversion units 2 (2a to 2n) are connected in parallel to be used as power output terminals for a large AC load. Alternatively, a plurality of AC loads are individually connected to the output terminals 4 (4a to 4n) of the plurality of the power conversion units 2 (2a to 2n).
FIG. 3 illustrates an example of a configuration of the power conversion unit 2a. Each of the power conversion units 2 (2a to 2n) is provided with three sets of half-bridge circuits which are configured, for example, by connecting six semiconductor switching elements Q1 to Q6 consisting of insulated-gate bipolar transition (IGBTs) or the like in such a manner that every two semiconductor switching elements are connected in series and inserted between a positive electrode and a negative electrode of the DC input terminal in parallel with each other. Each of the semiconductor switching elements Q1 to Q6 is driven to be alternatively turned on and off with a predetermined phase difference for each half-bridge circuit to switch a DC voltage applied to the half-bridge circuit. In addition, each of the power conversion units 2 (2a to 2n) is configured to connect serial connection points which are central points of the half-bridge circuits to the output terminals 4 (4a to 4n) so as to output a three-phase AC power having a predetermined voltage.
In addition, the reference letters D1 to D6 denote freewheeling diodes which are connected in reverse parallel with the respective semiconductor switching elements Q1 to Q6. In addition, the reference numeral 5 denotes a condenser which is arranged between the DC input terminals of each of the power conversion units 2 (2a to 2n) to stabilize the DC voltage applied to the power conversion unit 2 (2a to 2n). In addition, the reference numeral 6 denotes a control circuit which controls the semiconductor switching elements Q1 to Q6 to be turned on and off as described above. The power conversion device having such a configuration is described in detail, for example, in Patent Document 1 or the like.
In Patent Document 1 described above, switches 7 using mechanical contacts are provided as circuit breakers in the DC input portions of the plurality of the power conversion units 2 (2a to 2n). Furthermore, Patent Document 1 discloses a configuration where DC reactors 8 are installed in the DC link portion 3 with respect to the DC power supply unit 1. When a defect such as a short-circuit occurs in any one of the power conversion units 2 (2a to 2n), the switch 7 disconnects the defective power conversion unit 2 (2a to 2n) from the DC power supply unit 1. By doing so, only the remaining faultless power conversion units 2 (2a to 2n) can be operated.
However, until the defective power conversion unit 2 (2a to 2n) is disconnected by the switch 7, an excessive current flows from the other faultless power conversion units 2 (2a to 2n) to the short-circuit point of the defective power conversion unit 2 (2a to 2n). Due to the excessive current flow, the other faultless power conversion units 2 (2a to 2n) may fall into an excessive current state. The DC reactors 8 have a function of preventing the faultless power conversion units 2 (2a to 2n) from falling into the excessive current state by restricting the rise of the current flowing in the other faultless power conversion units 2 (2a to 2n) at the time of occurrence of the short-circuit fault.